In the MSC POOL networking, in some scenarios, such as avoiding over-load prevention, equipment upgrade and so on, network operators need to move the load of a certain MSC node in the MSC POOL to another MSC node in the MSC POOL, thus to realize load redistribution. In the load redistribution process, in order to achieve the purpose of reducing the impact on the terminal users and other network element entities as possible, the 3rd Generation Partnership Project (3GPP) protocol defines a “two-stage” implementation mechanism for the circuit switch (CS) domain, the specific process is as follows:
step 1: the operator uses the Operations and Maintenance (O&M) command to initiate a load redistribution process for a certain MSC node, and enters into the first stage;
step 2: in the first stage, when the user equipment (UE) performs the location update or International Mobile Subscriber Identity (IMSI) attach and the MSC receives a Location Update or an Attach Request sent by the UE, one Temporary Mobile Subscriber Identity (TMSI) including a null-Network Resource Identifier (Null-NRI), that is the Null-NRI, is distributed to the UE, moreover, a Non-broadcast Location Area Identity (Non-broadcast LAI) is included in the response message;
step 3: after the UE receives the non-broadcast location area identity, it uses the previously distributed TMSI to perform the location update immediately, since it is the Null-NRI contained in the TMSI, the Non Access Stratum (NAS) Node Selection Function (NNSF) entity module will route the UE to one new MSC node;
step 4: The first stage lasts two periodic location update cycles, thus ensuring that most of the UEs have completed at least one location update, and for those UEs which don't perform the location update due to service being kept continuously in the first stage, they will be processed in the subsequent second stage;
step 5: in the second stage, the MSC sequentially processes the UEs whose services are kept and initiates a TMSI redistribution process, and distributes a TMSI including the Null-NRI and a non-broadcast location area identity to the UE;
step 6: after completing the current service, the UE uses the newly distributed TMSI to perform the location update immediately because a non-broadcast location area identity is received, and since the TMSI contains the null-NRI, the NNSF entity module routes the UE to one new MSC node.
The main disadvantage of the aforementioned process is that the first stage has a long duration. Although the time length of the periodic location update cycle can be configured by the operator, it usually takes 30 minutes to 90 minutes. In some special application scenarios, such as when the mobile switching center is in need of emergent maintenance, hardware/software upgrades as well as a load redistribution process is initiated before exiting the service, and the load redistribution processing takes too long time, which might damage the timeliness of the maintenance.